Modulated Optical Reflectance (MOR) measurement is often used for ion implant metrology. MOR technology utilizes an intensity modulated pump laser beam to create carrier plasma and thermal waves in a semiconductor sample. A second probe laser reflects from the excited area and the changes in optical reflectance coefficient caused by the propagating plasma and thermal waves are recorded as the MOR signal. Commercial systems utilizing this technology are used in monitoring the ion implantation process used in semiconductor manufacturing. Techniques that can improve the signal-to-noise ratio for these measurements increase the value of the measurements and extend the range of applications for metrology tools utilizing MOR technology.
Prior art systems using MOR for ion implant metrology have been limited in signal-to-noise ratio by probe laser noise, and specifically by laser intensity fluctuations with frequency content near the modulation frequency of the MOR technique. The MOR technique leads to a tiny amount of modulation riding on a relatively large DC probe laser beam. The ratio of signal level to DC, laser intensity is typically 1 part in 104. Probe laser intensity fluctuations thus pose a significant obstacle to refining the precision and speed of MOR measurements. The probe beam fluctuations arise from various sources including interference and feedback phenomena interacting with the highly non-linear system of the laser cavity in which small perturbations can cause the energy of various cavity modes to fluctuate. Coupling between modes and the effects of temperature shifts can lead to “mode hopping”, a term describing an unstable balance of energy between different cavity modes where laser output fluctuations are enhanced. A laser specification known as Relative Intensity Noise (RIN) is a measure of these fluctuations that are a fact of life in commercial lasers that are available for use in measurement systems. These fluctuations may be many times higher in intensity than the Schott noise limit for a probe laser of the same average intensity.
These fluctuations occur at frequencies that are typically too high for correction by the use of typical normalization and standardization techniques. Prior art methods of dealing with these fluctuations in MOR systems have included techniques for reducing laser noise fluctuations in the laser. One of these techniques uses modulation of the probe laser at a very high frequency compared to the pump modulation to stir the laser diode modes and thus improve laser diode noise. Active power stabilizers have also been used to reduce laser intensity fluctuations, but operating them at high enough bandwidths to significantly reduce the fluctuations near the MOR modulation frequency is expensive and technically challenging.
It is within this context that embodiments of the present invention arise.